There are numerous components of industrial and utility equipment whose dimensional and geometric characteristics are of a critical nature and therefore must be manufactured to extremely strict specifications. A notable example of one such critical component is the flow channel of a nuclear fuel assembly or bundle, such as disclosed in U.S. Pat. No. 3,689,358. These channels are elongated tubular components of square cross section, which may measure approximately 6 inches on each side and on the order of 14 feet in length. Typically, the channels are created by seam welding two U-shaped channel sections together. The preferred material is a zirconium alloy, such as Zircaloy, on the order of 125 mils thick. It is imperative that these flow channels are manufactured to the proper dimensions and be free of geometric irregularities, such as face or side bulge, out-of-square cross sections, non-parallelism of sides, longitudinal bow and twist, and the like. Unfortunately, the channel creating step leaves residual geometric and dimensional irregularities, as well as residual stresses. Thus as a final manufacturing step, the channels must undergo a thermal sizing and annealing step to eliminate these irregularities and stress.
The thermal sizing-annealing step involves inserting an elongated, closed-fitting mandrel into the channel and raising the temperature of the channel to about 1100.degree. F. in an inert atmosphere. By virtue of the differential thermal expansion of the channel and mandrel, the mandrel expands into engagement with the channel, causing the channel to yield plastically to the specified final form. Typically, the outwardly directed mandrel forces are exerted solely on the four corners of the channel throughout their lengths. After a suitable anneal time, the channel is returned to room temperature and the mandrel is withdrawn, leaving the channel in a stable form substantially free of geometric irregularities and stress. It has been found that mandrel insertion is sometimes quite difficult due to the fact that certain irregularities produce sliding friction between the channel and the mandrel as the latter progresses into the channel interior. A particularly serious problem, however, is scoring of the interior corner surfaces inflicted during mandrel insertion, mandrel withdrawal, and/or relative movements of the mandrel and channel surfaces during the heat-up/cool down cycle due to differential thermal expansion. Such surface blemishes must be removed by abrasive techniques, resulting in wall thinning in the mechanically critical corners of the channel. If not removed, the blemishes may cause stress concentration sites during operation in a nuclear reactor core. They must therefore be avoided.
The thermal sizing apparatus of commonly assigned Wilks U.S. Pat. No. 5,027,635 is specifically directed to facilitating insertion and withdrawal of a mandrel into and from the interior of a channel involved in a thermal sizing-annealing process, while avoiding channel surface damage. The apparatus of this patent includes a die having four elongated die elements, which is initially inserted into a channel with the die elements respectively situated in coextensive, contiguous relation with the four corners of the channel. A mandrel equipped with a plurality of rollers is then inserted into the channel. The rollers are situated to make rolling contact with the die elements, thereby facilitating insertion as the die elements are pressed into the channel corners. Since the mandrel engages the stationary die elements rather than the mandrel during insertion and withdrawal, scoring of the channel inner corner surfaces is eliminated. When the mandrel and channel are heated to a suitable channel thermal sizing-annealing temperature, the mandrel expands at a faster rate than the roller journal mountings to the mandrel, such that mandrel bearing surfaces grow outwardly beyond the roller peripheries into thermal sizing engagement with the die elements. While the apparatus of this patent, in theory, achieves the desired objectives, it has several practical drawbacks. The mandrel design is relatively complex and extremely expensive to fabricate. Also, with repeated thermal cycling, the roller journals either develop excessive play or bind up. All too frequent adjustment and maintenance is thus required to maintain facile mandrel insertion and to avoid irreparable damage to the roller journals. If the channel is badly deformed, e.g., excessive bow or twist, mandrel insertion is difficult, even with the rollers.